Fiber Bragg gratings are widely used in the fields of optical fiber communication and fiber sensor systems. These Bragg gratings have a number of uses in fiber optics, notably as filters and reflectors. In a known process of fiber grating fabrication, all coatings must be stripped off before the grating can be inscribed or written and, in order to preserve the mechanical strength of the fiber, it must be re-coated soon after the grating is written. This method is time consuming and has the potential of reducing the fiber strength due to exposure of the bare fiber to the air.
To solve this problem in the art of inscribing fiber Bragg gratings, a number of solutions have been proposed. These include using a specially developed UV-transparent polymer coating, writing the grating using near UV light around 330 nm instead of at more conventional wavelengths, writing on-line as the fiber is being pulled, and using a specially developed coating which can be removed thermally, then immediately re-coating in an automated production system.
The polymer used in Espindola et al., Fibre Bragg Grating Written Through a Fiber Coating, although it has a lower absorption than the normal UV-curable polymer coating, still has a strong absorption at 248 nm or 257 nm. This increases the grating writing time and reduces the mechanical strength of the fiber when higher UV exposure power is used. In Starodubovet al., Efficient Bragg grating fabrication in a fibre through its polymer jacket using near-UV light, a specially developed phase mask is needed in order to operate at the non-standard wavelength. In addition, care is also required to control the average laser power in order to reduce damage to the coating. On-line production of fiber gratings, such as that described in Dong et al., Single pulse Bragg gratings written during fibre drawing, is limited to the manufacture of gratings which can be made with a single laser pulse.
The method proposed in Singh et al., Automated In-Line Production of Fibre Bragg Grating Using Special Coatings, although attractive, requires both a special coating and an automatic production system. The thermally strippable coating also has a low heat resistance in many applications.